Predicate Logic

Let L be a language which has two action constants: which has "4 spots" and which has 10 spots.
A. Define by recoursion the numbers of commas in a noun of L.
B. Prove by using induction on the construction of a noun, that in every noun, the number of commas can be divided by 3 .

Hope you'll be able to help me...I've no idea about these two questions
Thanks !